vt: Fix frequency calcuation for bell

386BSD provided a MD function sysbeep. This took two arguments (pitch
and period). Pitch was jammed into the PIT's divisor directly (which
means the argument was expected to sound a tone at '1193182 / pitch'
Hz). FreeBSD inherited this interface.

In commit e465985885 (svn 177642, Mar 26 2008), phk changed this
function to take a tone to sound in hz. He converted all in-tree
instances of 1193182 / hz to just hz (and kept the few misguided folks
that passed hz directly unchanged -- this was part of what motivated the
change). He converted the places where we pre-computed the 8254 divisor
from being pitch to 1193182 / pitch (since that converts the divisor to
the frequency and the interfaces that were exposed to userland exposed
it in these units in places, continuing the tradition inherited from SCO
System V/386 Unix in spots).

In 2009, Ed Shouten was contracted by the FreeBSD Foundation to write /
finish newcons. This work was done in perforce and was imported into
subversion in user/ed/newcons in revision 199072
(https://svnweb.freebsd.org/base?view=revision&revision=199072) which
was later imported into FreeBSD by ray@ (Aleksandr Rybalko).

From that earliest import into svn import to this date, we ring the bell
with:
      sysbeep(1193182 / VT_BELLPITCH, VT_BELLDURATION);
where VT_BELLPITCH was defined to be 800. This results in a bell
frequency of 1491Hz, more or less today. This is similar to the
frequency that syscons and pcvt used (1493Hz and 1500Hz respectively).
This in turn was inherited from 386BSD, it seems, which used the hard
coded value 0x31b which is 795 -> 1500Hz.

This '800' was intended to be the bell tone (eg 800Hz) and this
interface was one that wasn't converted. The most common terminal prior
to the rise of PCs was the VT100, which had an approximately 800Hz
bell. Ed Shouten has confirmed that the original intent was 800Hz and
changing this was overlooked after the change to -current was made.
This restors that original intent and makes the bell less obnoxious in
the process.

Reviewed by:		des, adrian
Differential Revision:	https://reviews.freebsd.org/D32594
Sponsored by:		Netflix

sys/dev/vt/vt_core.c

@@ -121,7 +121,7 @@ const struct terminal_class vt_termclass = {
 
 /* Bell pitch/duration. */
 #define	VT_BELLDURATION	(SBT_1S / 20)
-#define	VT_BELLPITCH	(1193182 / 800) /* Approx 1491Hz */
+#define	VT_BELLPITCH	800
 
 #define	VT_UNIT(vw)	((vw)->vw_device->vd_unit * VT_MAXWINDOWS + \
 			(vw)->vw_number)